Azeotropic distillation of hydrocarbons



Patented June 5, 1951 AZEOTROPIC DISTILLATION OF HYDROCARBONS George R.Lake, Long Beach, Calif., assignor to Union Oil Company of California,Los Angeles, Calif., a corporation of California No Drawing. ApplicationJanuary 12, 1946, Serial No. 6't1,009

1 Claim.

This invention relates to the preparation of pure hydrocarbons frompetroleum, and particularly the separation of naphthenic hydrocarbonsfrom paraffinio hydrocarbons. These hydrocar: bons when contained in ahydrocarbon mixture of relatively narrow boiling range are virtuallyimpossible to separate by ordinary fractional dis tillation and thepresent invention contemplates their separation by means of azeotropicdistillation in the manner as hereinafter set forth. This invention is acontinuation-in-part of my ccpending application Serial No. 412,814, nowabandoned. a

An object of the present invention is to further the progress inpreparing pure compounds from a heterogeneous petroleum mixture, usingin this particular case a method which involves fewer steps than achemical method and which yields a purer product than that produced bycareful fractional distillation and/r extraction with selectivesolvents.

Another object of the invention is to prepare from a given fraction ofpetroleum, such as gasoline, kerosene, or a narrow boiling rangehydrocarbon fraction prepared from such materials, these fractionsconsisting of a mixture of parafiinic, isoparaffinic, naphthenic,olefinic and aromatic hydrocarbons, a fraction that is essentiallyparafiinic or isoparaffinic or naphthenic in character.

A. particular object of my invention is to separate naphthenichydrocarbons from parafiinic hydrocarbons by distilling the complexhydrocarbon fraction in the presence of a saturated heterxocyclicorganic compound having four or more atoms, particularly one in which atleast one of the atoms in the ring is oxygen, sulfur or nitroin suchmanner that the partial vapor pressure orfugacity of at least onecomponent in the fraction is changed sufficiently to permit itsseparation by controlled fractional distillation. This type offractional distillation will be referred to hereinafter as azeotropicdistillation and the substance or substances which are added to thefraction which effect the aforementioned chang will be referred to asazeotrope formers.

According to my invention, the separation of a specific hydrocarbonorhydrocarbon fraction from a mixture of hydrocarbons is accomplished byazeotropic distillation whereina saturated heterocyclic organic compoundhaving four or more atoms is added to the petroleum fraction and themixture is subjected to controlled .fractional distillation. Theaddition of the azeotrope former to the petroleum fraction results informing a more volatile azeotrope with certain of the hydrocarboncomponents which may thenbe distilled from the remaining hydrocarboncomponents. Thus when it is desired to segregate naphthene hydrocarbonsfrom paraiiin hydrocarbons, the fractional distillation of this mixtureto which an azeotrope, former has been added results in the formation ofan azeotrope consisting of the paraffin hydrocarbons and the azeotropeformer which is more volatile than-the naphthene hydrocarbons either inthemselves or in azeotropic mixture with the azeotrope former employed.Thefractional distillation of the mixture results in distilling overheadthe paraffin hydrocarbons in admixture with the azeotrope former leavingthe naphthene hydrocarbons as undistilled bottoms, While it is preferredto per form the azeotropic distillation on a fraction containingpredominantly naphthenic and paraffinic hydrocarbons and thus effect thedistillation in such a manner that one of the components-in thefraction, 1. e., the naphthenic hydrocarbons remain in the substantiallypure form as Idist'illation bottoms, the process of the presentinvention may be employed on a hydrocarbon fraction containing as wellas paraflinic and naphthenic hydrocarbons, aromatic hydrocarbons andolefinic hydrocarbons. If such be the nature of the hydrocarbonfraction, a separation between the naphthenic and the paraffinichydrocarbons, according to the present invention, may still be performedin which casethe naphthenic residue from the distillation will becontaminated with aromatic and olefinic hydrocarbons. The naph thenichydrocarbons, however, may be separated therefrom by continuing theazeotropic distillation \vtih the azeotrope, formers herein employed orby subjecting theinaphthenic rich residue to a second azeotropicdistillation with the present azeotrope former or with other suitableazeotrope formers.

In such cases where the hydrocarbon fraction contains more than twocomponents of different chemical characteristics, as for examp1e,aro-'-matics, naphthenes and parafiins, and it is de 3. sired to separate oneor more of these components from the other component or components, theseparation may be accomplished by stage azeotropic distillation toremove first one component and then another component. For example, anazeotrope former, such as dioxolane, may be added to a mixture ofaromatics, naphthenes'and paraffins having a boiling range ofapproximately 200 F. to 240 F. and the mixture then distilled to removeas overhead fractions, first an azeotrope of the paraffins withdioxolane and then an azeotrope of the naphthenes with more dioxolaneleaving the aromatics as undistilled. bottoms which may or may notcontain dioxolane. The point at which one component, the paraifins, forexample, is substantially completely distilled from the remainingcomponents may be observed by a rise in distillation temperaturenecessary to effect further distillation of the material in the still.Thus, in the above example, the distillation is initially carried out atan overhead temperature of approximately 162 F. at which temperature theparalfin hydrocarbons together with azeotrope former distill from theremaining hydrocarbon components. --When substantially .all'of -theparafiin components have been evaporated from the mixture, it will benecessary to raise the distillation temperature so that the overheadtemperature will be increased to, for example, approximately 164 F. inorder to effect further removal of hydrocarbon components. This increasein temperature indicates that substantially all of the parafiins werepreviously distilled from the mixture and that the next hydrocarboncomponents, for example the naphthenes,

'former C there is an azeotrope formed between A and C and also betweenB and C, which azeotropic mixtures may have no greater boiling pointspread than existed between the original components A and B. However, ithas been shown many times over that in the majority Of cases azeotropesformed with components A and B by the azeotrope former C are morereadily separated by distillation than are components A and B in theabsence of az'eotrope former despite the same or even smallerdifierences in'boiling points existing between the azeotropes. I havefound this to be true in many instances of azeotropic distillation ofnaphthenes and parafiins with the a'zeotrope formers of the presentinvention. It should be emphasized therefore that I do not wish to limitmy invention to the usage of azeo trope formers which form an azeotropewith only one component in the hydrocarbon mixture, nor to the usage ofazeotrope formers which form azeotropes with more than one component inthe hydrocarbon mixture which azeotropes boil no further apart than thecomponents themselves.

While the invention is adapted to, the separation of hydrocarbons ofcharacteristics different from each other I have. found that thisprocess particularly useful for producing naphthenic hydrocarbons suchas methylcyclohexane, dimethylcyclohexane, dimethylcyclopentane, and

the like, having a high degree of purity from gasoline fractionsproduced from straight run or synthetic gasoline, such as those producedby cracking, polymerization or reforming. The production ofsubstantially pure naphthenic hydro? carbons is becoming increasinglyimportant as a result of the expanding usage of naphthenic hydrocarbonsas intermediates in the synthesis of organic chemicals suchas adipicacid, succinic acid, and the like.

Besides dioxolane mentioned above, other saturated heterocyclic organiccompounds may be employed asazeotrope formers. These include the sixmembered compounds in which at least one of the atoms in the ring isoxygen, nitrogen .or sulfur, such as oxane, trioxane, piperidine,

thiane, dimethyl dioxane, morpholine, thioxane,

piperazine, dithiane, thioformaldehyde, etc., the derivatives of suchsix membered compounds, such as dibutanol, n-ethyl piperidine, n-methylmcrphoiine. n-morpholine ethanol, n-phenyl morpholine, pentamethylenesulfone, etc., the five membered compounds in which at least one of theatoms in the ring is oxygen, nitrogen or sulfur, such as oxolane,pyrrolidine, thiolane, dioxolane, methyl dioxolane, etc., and thederivatives of such five member-ed compounds such as n-ethylpyrrolidine, tetra-methylene sulfide,-tetrae hydrofurfuryl alcohol,etc., the four membered compounds, such as trimethylene sulfide, trimethylene oxide, trimethylenimine, etc., and the derivatives of suchfour membered compounds, such as trimethylene sulfone, n-ethyltrimethyh' enimine, etc. 1

Of the above-mentioned azeotrope formers. .I have found the heterocyclicoxygen containing compounds and particularly dioxane, dioxolane, andtrioxane are very eiiicient azeotrope formers for separating anaphthene-parafiin hydrocarbon fraction having a narrow boiling rangepreferably not more than 50 F. into a naphthenic and parafiinic richfraction. The azeotrope former should be chosen with respect to theboiling point of the hydrocarbon fraction, the higher the boiling pointof this fraction the higher will be the boiling point of the azeotropeformer capable of eiiecting the azeotropic distillation. Theboilingpoint of the azeotrope former should be not more than about 35 C.above 0r35 C. below the average boiling point of the hydrocarbonfraction to be separated. In other words the hydrocarbon fraction to beseparated should have an average boiling point not more than 35 C. aboveor more than 35 C. below the boiling point of the specific azeotropeformer to be'employed for its A separation. Thus, hydrocarbon fractionsto be separated with dioxolane, for example, as the azeotrope formershould have an average boiling point between 39 C. and 109 C., dioxolanehaving a boiling point of 74 C. In this regard, I have found that inmost cases it is preferable to employ an azeotrope former boiling belowthe hydrocarbon mixture for it is found that in this manner a greaterazeotropic spread is obtained. However, this is not always true forexample dioxane boiling at 214 F. is an effective azeotrope former forseparating n-heptane boiling at 208 F. from methylcyclohexane boiling at21.3.5 F. Further, in this respect I have, found, that the greatestazeotropic spread is obtained when employing an azeotrope ,formerboiling. in the lower limits of this lower temperaturerange; However,there must also be considered in em. ploying such an azeotrope formerthe fact that the greater boiling point .difierential between the mllllllilullwww azeotrope former and the hydrocarbon fraction, the greaterwill be the amount of azeotrope former necessary to accomplish a givenseparation. It is further necessary to balance these two opposingfactors in determining which azeotrope former is to be employed, and inthis regard consideration of the composition of the hydrocarbonfraction, the availability of the azeotrope formers and the like must betaken into consideration. Thus, in a fraction comprised predominantly ofnaphthenic hydrocarbons containing only a minor amount of paraffinichydrocarbons as impurities it may be economically practical to employ anazeotrope former boiling some 25 to 35 C. below the average boilingpoint of the hydrocarbon fraction; whereas, in a fraction comprised ofapproximately equal proportions of naphthenic and parafiinichydrocarbons it may be deemed advisable to employ an azeotrope formerboiling in the range of C. to C. below the average boil ing point of thehydrocarbon fraction. These generalizations concerning the selection ofthe azeotrope former to be employed are intended only to be helpful inthe utilization of my invention but should not be considered as limitingits application in any way whatsoever inasmuch as I may use as azeotropeformers compounds of the above identified type boiling not more thanabout C. below the hydrocarbon fraction, and not more than about 35 C.above the hydrocarbon fraction.

The type of distillation to be used depends somewhat on the quantity ofthe aforementioned azeotrope former employed. I may take any portion ofazeotrope former to the petroleum fraction that I desire, depending uponthe efficiency of the operation or the purity of the product desired,and the technique to be used in the distillation. The proportion of theazeotrope former may be readily adjusted to an ideal point, againdepending on whether I desire the portion high in naphthenic hydrocarboncontent to remain as bottoms in the distillation in practically purestate, i. e., free from paraffinic hydrocarbon, or whether I wish todistill a portion of the parafiinic hydrocarbons leaving a portionthereof as bottoms together with the naphthenic hydrocarbon. Also thedistillation temperature and amount of azeotrope former may be adjustedto effect the distillation of all of the paraflinic hydrocarbonstogether with a portion of the naphthenic hydrocarbons. In other words,the desired efficiency of separation of the naphthenic and parafiinichydrocarbons is dependent upon the proportion of azeotrope formerselected, the efficiency of the tillation column and similar factors.

rther, I have found that in conducting the distillation in a batchwisemanner it is preferable to introduce the azeotrope former in incrementsduring the course of the distillation for in this manner there isavoided the loss of any uncombined azeotrope former in the distillate,which normally occurs if the entire quantity thereof is initiallyemployed. By permitting the attainment of an equilibrium in thecomposition of the residue in a continuous distillation the azeotropeformer may be added incremently in such a distillation whereby the samebeneficial effect is obtained as in the batchwise operation.

In order to separate the azeotrope former from the azeotropicdistillate, it is merely necessary to extract the condensate mixturewith a solvent adapted to extract or dissolve the azeotrope former andsubstantially none of the hydrocarbons. By allowing this mixture tosettle, two dis- .6 tinct layers are formed, an upper layer consistingof the hydrocarbon and a lower layer of azeotrope former dissolved inthe solvent. Solvents useful for the purpose include th nitroparaifins;such as nitromethane, nitroethane, nitropropane, propylene glycol anddiethylene glycol, and even the saturated heterocyclic organic compounds,having different boiling points than the hydro carbons to be separatedfrom the azeotropic distillate. In some cases, the separation ofthe'azeotrope former from the hydrocarbons may be accomplished bycooling the azeotropic distillate sufficiently, as for example, below 70F. in order to reject-the hydrocarbons from the azeotrope former. Mostof the azeotrope formers disclosed herein are water soluble and theseare preferably extracted from the azeotropic distillate with water atanappropriate temperature to effect the desired result. The azeotropeformer may be recovered from the non-aqueous solvent or water by simpledistillation, the overhead being either the azeotrope former or thesolvent depending upon the relative boiling points of these twomaterials.

Other objects, features and advantages of my invention. will becomeapparent to those skilled in the art from the following examplesthereof. However, it will be observed that these examples are not to betaken as limiting my invention since the process is applicable toseparate other complex hydrocarbon fractions employing the otherazeotrope formers disclosed herein for effecting the desired separation.

EXAMPLE I A blend of equal parts of methylcyclohexane and n-heptane wasprepared and a portion of this blend was subjected to simple fractionaldistillation while another portion was subjected to an azeotropicdistillation in the presence of dioxolane as the azeotrope former. Bothof these distillations were made on a 64 plate still at an internalreflux ratio of 64 to 1. The results of the simple distillation aresummarized briefly in Table 1 below:

The other portion of the methylcyclohexanen-heptane fraction wasdistilled in the presence of two parts dioxolane, the results of thedistillation being shown in Table 2 below:

TABLE 2 Distillation of 50-50 mixture of n-heptane and methylcyclohemanein the presence of two volumes dioxolane Volume M. O. H. N -heptaneFraction Per Cent Vol. Per Vol. Per 3 of Charge. Cent Cent Initial 36.04. 4 95. 6 162 Intermediate 18. 5 32. 6 67. 4 l62164 Bottoms 45. 5 96. 04. 0

! Calculated on a dioxolane free basis.

It is immediately apparent that heptane and methylcyclohexane cannot beefficiently separated by'simple-fractional distillation in a column ofthis character; whereas, a high. degree of separation is obtainable byan azeotropic distillationin the presence of dioxolane.

EXAMPLE II had been distilled together with dioxane, leaving about 30%of the stockv in the still. After separating the diozane from thesefractions thus produced, bywashing with water, the hydrocarbons.recovered in the first distillation had a gravity of 72 A. P. I.indicating about 90% by volume of paraflinhydrocarbons, the;hydrocarbons recovered from the second distillation had a gravity of 63A. P. I. indicating about equalvolumes of parafiin and naphthenehydrocarbons and the distillation bottoms had a gravity of'52A. P. I.indicating about 95 by volume of naphthene hydrocarbons.

The. foregoing description and eXamDle-sotmy invention are not intendedto be taken; as limiting my invention but only as illustrative thereofsince many variations may be madelby those skilled in the art withoutdepartingfrom the spirit or scope of the following claim:

I claim;

A process for the separation of heptane and methcyclohexane whichcomprises azeotropically distilling said methylcyclohexane and heptanemixture in the presence of a suflicient amount of dioxolane to vaporizethe heptane contained in said mixture together with said dioxolanethereby leaving said methylcyclohexane in the residue.

GEORGE R. LAKE.

REFERENCES CITED The following references are of recordinth file of thispatent:

UNITED STATES PATENTS Number Name Date 2,313,537 Greenburg Mar, 9,19432,352,534 Greenburg June 27, 1944 2,367,701 Tooke J an.23, 19452,368,050 Tooke Jam-23, 1945 2,368,597 Morris et a1. Jan.' 30, 19452,397,839 Clark Apr. 2, '1946 OTHER, REFERENCES Mair et a1 Bureau ofStandards Journal of Re search, vol. 27, pages. 45, 44, 46, and 49through 54.

